Material Resists Bacteria, Prevents Infection, Saves Lives

Engineers at Virginia Tech have created a new methodology for building bacteria-resistant surfaces for medical devices and other applications that prevent infection and could save lives.

by Lina Zeldovich

January 14, 2019

Engineers at Virginia Tech have created a new methodology for building bacteria-resistant surfaces for medical devices and other applications. In the past, many research teams investigated anti-bacterial properties of various materials and devices, but this study presents a mathematical and biophysical model that can help create anti-bacterial coatings to spec, aimed to repel a particular pathogen based on its biological characteristics.

Known as biofilms, microbial communities that grow on the surfaces of medical devices such as urinary or vein catheters can cause life-threatening infections, posing significant risks to patients’ health. In the United States, these healthcare-related infections add up to $45 billion in additional healthcare costs and cause 100,000 deaths a year. About 70 percent of these infections arise from the microbial biofilm that develops on catheters. Microbial biofilms also present problems in industrial settings, such as wastewater treatment plants and other applications.

Microorganisms attach to surfaces in two steps, said Bahareh Behkam, an associate professor of mechanical engineering at Virginia Tech’s School of Biomedical Engineering and Sciences, who co-authored the study. First, they use intermolecular forces such as electrostatic attractions to get close to the surface, and then they employ their biological “tools,” including adhesive proteins or special hooks.

In order to attach to the surface, an organism must expend energy, and some surfaces require more energy spending than others, with surface roughness being a key factor. Very smooth surfaces are harder to attach to just like a very slippery road is hard to walk on. Rough surfaces with many “big bumps” are also difficult to attach to because the microorganism must stretch itself to wrap around them, which is an energy expensive operation. “The larger the required energy change, the less desirable a surface is for an organism,” Behkam said.

But covering a surface of a catheter or other devices with nanofibers of the right diameter and spacing can alter its roughness to be maximally unfavorable for microbial cells to cling to, which can prevent biofilm formation.

Different microorganisms have different rigidity, causing some to stretch better than others. That means the surface’s nanofiber design must be built to specifically repel the organism that tends to adhere to such devices. But testing hundreds of possible nanofiber arrangements and surface textures against a varied pathogen community is a laborious and expensive process. To overcome that, the team created a mathematical model that lets users input the parameters of a specific microorganism along with the material used for a particular application. It then calculates the surface texture’s size and spacing that would repel this pathogen in the optimal way.

“The underlying material and the organism both get fed into the model and the model tells you what fiber diameter and fiber spacing you should coat your material with,” Behkam said.

The team tested their model on a common yeast, Candida albicans, which in its pathogenic form can cause fungal infections inside the urinary tract and other places in the body. Once settled, the pathogen grows sharp filaments and pierces the tissues. It can enter the bloodstream through those holes, resulting in systematic infections and deaths.

“It’s very common in the elderly, because they may not feel any discomfort,” Behkam says, adding that C. albicans infections often go unnoticed, but “once it gets into the blood, it’s very deadly.”

The team calculated that C. albicans would have a hard time attaching to the surface built with fibers of 1.2 micrometers in diameter and spaced at 2 micrometers each—and built a test surface to test their hypothesis. Using a polystyrene polymer, the team created nanofiber threads and spun them onto a tubular surface that would serve as a catheter’s coat. “If you dip your finger in honey and pull it up, you will create a long filament,” Behkam said. The honey filament, however, would eventually thin and break. Under the right conditions, the polymer sustains the thread, which can be spun at the required intervals.

The team tested their polystyrene nanofiber-coated surfaces and found the model accurately predicted the reduction in C. ablicans colonization. Regardless of the materials the catheters were built from—such as polyurethane, latex, or silicon—the nanofiber texture built with the study’s model optimally reduced microbial attachment.

Rigoberto Advincula, professor of macromolecular science and engineering at the Case Western Reserve University, who was not involved in the research, finds the idea promising.

“What’s interesting in this study is that they try to quantify the adhesion effect based on fiber orientation, thickness, different types of surface energy and even statistical analysis,” he said.

Advincula thinks the model can be further improved by taking into account different molecular cues and surface chemistry, and could be put to use by manufactures designing bacteria-repellent materials. He also sees other possibilities besides medical uses.

“One viable application I can see is on packing materials,” he said. “Polystyrene and other plastics can be put on the packaging surfaces to help to resist bacterial biofilm and colonization.”